Something deeper...

[Sora341]

So there are many resemblances of Riku and Xehanort, and those directly connected to him. Putting aside that they both have grey hair, there are many other factors that I have noticed.

Remember in the first Kingdom Hearts, when you fought Riku on Destiny Islands, and he would stand there blocking with his sword, saying: "Giving up already?" Inconsequential, Right?

Well remember, Vanitas, who was made directly by Xehanort... said almost the exact same statement when he had taken over Ventus's body after his an Aqua's fight. As Aqua sat on the ground panting, Vanitas/Ventus exclaimed... "What's wrong, giving up already?"

Yet that was when Riku was only a child...

In dream drop distance, when YX was putting Sora into another dream/vision, he beckoned to Sora, and in that instance his image changed into Riku, beckoning him as he did before Destiny Islands fell to Darkness...

Could this mean anything, or is this direct symbolism?

 

[Ruran]

Terra also says the "giving up already" line to Ven in a flashback.

I think it's more symbolism/misdirection than anything because at the time we didn't know what Vanitas looked like or who he was connected to. On the surface, he appeared to have a connection with Riku and shared a similar relationship to Ven as Riku and Sora did with each other, but it later becomes apparent that they have little to nothing to do with each other outside of a mutual relationship with Xehanort.

 

[Antifa Lockhart]

It's just a parallel. If you think they're long-lost father and son, I have some disappointing news for you.

(For the record, Riku has ALWAYS treated Sora that way, we see it happen when Aqua visits the islands, Riku says the exact same thing to him and he was five.)

 

[Gram]

Its definitely just symbolism, this series is riddled with it. Family has not and likely never will worry about family. We know characters have family, Riku mentions parents, we see Kairi's grandmother, we hear Sora's mother and Riku mentions Sora's father during BBS.

As for Riku and Xehanort, just think carefully about what your pointing out. Not only do they parallel but so does Riku and Terra just like Ventus-Vanitas parallels Sora-Roxas to an extend.
It's all symbolism.

Xehanort represents unbound curiosity. The one who embraced the darkness from the start and now uses it freely.
Terra represents the opposite. Even though he knew he had it, he resisted that darkness till the end yet still lost to it.
Riku prepresents redemption to Terra and Xehanort's falling to darkness. Riku, also desperate to leave the islands, jumped headlong into darkness and then later spend the series overcoming it which is something Terra couldn't do and something that Xehanort doesn't wish to do.